In order to improve the channel estimation accuracy, [1]} further proposed the discrete Fourier transform (DFT) protocol based channel estimation scheme, where all RIS elements are always turned on. In this scheme, the entire cascaded channel estimation process is still divided into \(N\)  stages. However, in each stage, the reflecting vector \(\mathbf {\theta }\)  at the RIS is specially designed as one column vector of the DFT matrix. After \(N\)  stages, based on the LS algorithm, the cascaded channel \(\bf {H}\)  can be directly estimated based on all received pilot signals in \(N\)  stages at the BS. It is noted that the overall reflecting matrix for \(N\)  stages forms a DFT matrix of size \(N\times N\) , which has been proved to be the optimal choice to ensure the channel estimation accuracy [1]}.
